I think you'll find serial number on the frame underneath the cylinder crane rather than on the grip post. On my later model 12 the serial number is on the right of the frame like all later models, but early 12's it was under the crane like S&W I think.
Good luck on finding dates for serial numbers.
you might check to see if your firing pin channel is crdded up, then make sure the innards are lubricated so nothing is keeping the hammer from falling properly. Or you could just have a weak mainspring or someone has clipped a coil too many.
